Metal surface treatment



Patented May 11, 1954 ceased, la

. Ur y C fton- I intQn, de Baltimore, 'Md.', by, ltolfe" Pottf berg, admin trator, Baltimore,' Md., assignors',

by direct and mesne assignments," eor reeporr Sulphur Company, New Ybrk, N." Y6,- ajcorpoe r'ationof. 'Delzmiarei- 0 D wi g Ori a rrl q t on une. 2,. 944... Serial No; 538,536, now "Patent N 2,538,792,"

deficit-lemurs; 16.11951. Divided and this'ap-' plication April'...6 .v1950 sqifilrrNbi 2 4 2 Claim 1.

This invention relatestdan improvedgllrocess for. the cleaning, descalingand deoxidizingof metals, more particularly. the. ferrous. alloys usually referred to.- asstainless. steels I'm present practice, theseoperationsl areiusuallyrcarrie ut by imm'rsingthemetal; in. strong. acids. wh operate by. attacking the... metal itself, and; so bringing aboutadetachment of the ,surfacesfiale oroxide.

The object of.v this. invention is to provide a treating bath directly acting. on the scale. or oxide itself.

According. to this invention, the, oxide;. ooating onth'e metal isdissolvedor converted in easily soluble form. by. immersion, in a bathof :a fused compound. containingappropriate ingredientsto bring about. this conversion- For. exam le. a ferrous alloy containing iron, nickel and chromium "may beeffectively treated by the. useof a bath of fused sgdium. hydroxide.containing an oxidizing agent such as sodium nitrate or sodium peroxide. This treatment removes cl 1; the whole, 'of the scale or oxide, or any rem or as to be r ead 91l b by a. short treatment with a weak'solution of old.

Oxide films on hot rolled chromiu" -el alloysteel may be removed by' treatment Q of' fused sodium hydroxide containing oxidants in minor amount usually less than 10%, for instance, as follows:

(a) Sodium nitrate, the sodium nitrate liberating oxygen according to the reaction 2NaNO3 02 the resulting films being easily removed in a I subsequent short acid treatment.

(bl Sodium peroxide (Nazoz), plus a subsequent short acid treatment.

These baths in which further oxidation of some or all of the components of the oxide film occur, permit reaction between one or more of the metal oxides and the composition forming the fused bath, with complete or partial conversion, so that upon removal any residual film exists in a form which, possibly due to the removal of some of its original component oxides, and partly due to the changed form of the remaining compounds, is removed much more readily with acids.

There is thus combined with a fused bath I 2 alkaline in. character. a further. oxidizing. efi a favorable to the formation of higher oxides.

In. certain cases it is; advantageous. to. give material a preliminary. treatment. n nacidsglution, 1.0% sulphuriog..or.nitrioacid.beingparticuy. effe e pplication asl rt. eriodpff; wo to five minutes after whichit, is washedoff in hot-.watendrying, before immersion inthe fused compound.

The. fusedbathwmay. con ist of. aconi ound o compounds. of. thedesired basic Properties, such as NaQHor-Naz Om. ay be. om o d ar ly of other suitable. fusible, substanoes which act as inert vehicles for a requisite. percentage; of such compound. or compounds. Such reaction products... as,v are, not soluble. in the, fused, bath its lt. areremoved bysubseq ent reatmeme fi r removal, from. the bath,

The. reaction.oompoundsu u lr armed when they are not soluble in the bath itself, m af 9w one. for... more ofhe.follow n ,oha g e is q (1'). re ecomposed;- be r: are soluble. Water.-

(2) Theyaremuchmore readily reagtedupon byacids. han the. rie naleqx deori oxi es,

The. oxidi in Qond tionsl oi he, ils d bath cambe produc in va usways, Forex (1:) By.- ha. sicl tiq xidieinsfsnb such. as. nitrates emanates... newness. 7 rates, persulphates, etc., principally as saltso f the alkali or alkaline earth metals;

(2) By formation by electrolytic means in the fused bath of oxidizing compounds, or of oxygen itself. For example, under controlled electrolytic conditions the formation of NazOz in fused baths of NaOH can be promoted. For instance, oxygen may be generated in the molten bath by passing of electric current between electrodes therein, as explained in Castner Patent 452,030 of May 12, 1891.

(3) Oxygen in gaseous form.

The oxidant in the fused bath has proven very effective and the resulting compounds are either directly soluble in the bath or are more easily removable by after-treatment than the oxides in their original form.

The numerous advantages of the process as herein described are of great economic importance when considered in comparison with standard commercial present day practices of descal- 3 ing, pickling and metal oxide film removal in general. Some of the more outstanding advantages are now briefly pointed out:

(1) The great reduction in time necessary for the cleaning operation by our process over the acid pickling process. The cycle of operations in our process can be completed in substantially under ten minutes. For example, in cleaning Ni-Cr alloy steel, an illustrative complete cycle for satisfactory oxide removal is as follows:

(a) 6-7 minutes fused NaOH+% NaNOa 900 F.; or 4-5 minutes at 950 F.

(12) Water quench, then (0) Treatment in HCl at 180 for 1 minute followed by a second treatment in 15% HNOs at 180. In the normal acid pickling process or combination acid processes, as applied to similar steels, the treatment time can run, for example, as high as 4 hours in 17 I-lzSO4 at 180 F.

(2) The normal acid pickling process leads to ing certain stages of their fabrication into the customary forms required for industrial needs, such as billets, castings, sheet, rod, wire, etc. For example, heavy oxidation takes place during the annealing process, and this oxidized surface layer has in most cases to be removed before further fabrication or processing can be carried out.

The present process accomplishes this quickly and efficiently and is thus of wide scope and great practical utility.

This is a division of our pending application,

, Serial NO. 538,536, filed June 2, 1944, Patent No.

a great loss of metal by the acid attack on the 1 base metal itself, this loss sometimes amount-- ing to 60 pounds of metal per short ton, whereas tests made on comparative metal loss caused by our process indicates actual metal sacrifice to fall substantially below 10 pounds per short ton.

(3) The great problem of spent pickling acid disposal is practically eliminated by the use of our process, the actual acid requirements and consumption for the subsequent acid treatment in this process being very small.

(4) The fused bath utilized is very reactive with carbon, oxidizing it to removable products. Carbon is present in most annealed metal oxide films and is extremely difilcult to remove by acid pickling. In these it is only thus completely removed by solution of the metal surrounding and beneath it.

(5) The short subsequent acid treatment re-- quired as an adjunct to the process diminishes the etching of the metal surface which is produced by the sustained action of acids over a long period as is normally required in acid pickling.

(6) The low cost of the chemicals required.

(7) The low cost of operation of the process.

(8) Simplicity of operation and of ingredients.

The metal alloys which are of general commercial application, and to which the invention can be beneficially applied, such as those of iron, chromium, nickel, manganese, cobalt, molybdenum, vanadium or tungsten, etc., in various combinations, are oxidized on the surface dur- 2,538,702 of January '16, 1951.

We claim:

1. A process for the treatment of surface oxide films on a metal article for the purpose of removing surface oxides therefrom comprising subjecting the surface oxides free of electric circuit connection to the action of a fused inorganic salt of the group consisting of the alkali and alkaline earth metal hydroxides and carbonates and containing oxygen in gaseous form, and continuing said treatment to the modification of the oxides to a form easily removable in acids without depletion of the treated metal, and then treating said surface in an acid bath to remove remaining modified oxides.

2. A process for the treatment of surface oxide films on ferrous metal article surfaces for the purpose of removing surface oxides therefrom comprising preliminarily treating the surface oxides in an acid solution, and then subjecting the surface oxides to the action of a fused inorganic salt of the group consisting of the alkali and alkaline earth metal hydroxides and carbonates and containing oxygen in gaseous form,

and continuing said treatment to the modification ofthe oxides to a form easily removable in acids Without depletion of the treated metal, and then treating said surface in an acid bath to remove remaining modified oxides.

References Cited in the file of this patent 

1. A PROCESS FOR THE TREATMENT OF SURFACE OXIDE FILMS ON A METAL ARTICLE FOR THE PURPOSE OF REMOVING SURFACE OXIDES THEREFROM COMPRISING SUBJECTING THE SURFACE OXIDES FREE OF ELECTRIC CIRCUIT CONNECTION TO THE ACTION OF A FUSED INORGANIC SALT OF THE GROUP CONSISTING OF THE ALKALI AND ALKALINE EARTH METAL HYDROXIDES AND CARBONATES AND CONTAINING OXYGEN IN GASEOUS FORM, AND CONTINUING SAID TREATMENT TO THE MODIFICATION OF THE OXIDES TO A FORM EASILY REMOVABLE IN ACIDS WITHOUT DEPLETION OF THE TREATED METAL, AND THEN TREATING SAID SURFACE IN AN ACID BATH TO REMOVE REMAINING MODIFIED OXIDES. 